South Korea's Mirim Lee birdied five of her last seven holes to take a three-shot lead at the US Women's Open.
More difficult conditions in the afternoon meant three of the world's top four - Lydia Ko, Brooke Henderson and Lexi Thompson - struggled.
England's Jodi Ewart Shadoff is tied for 11th place after a two-under 70.
"The course is perfect now," Lee told reporters as she finished.
Teenagers Ko and Henderson - the world number one and two - went round the course with world number four Thompson. World number three Inbee Park of South Korea is missing with a thumb injury.
The trio faced stronger winds and firmer greens and finished with scores of 73, 76 and 74 respectively.
The 2007 winner Cristie Kerr went round with Lee and is among the players in second place, three shots behind the leader.
"We played in the morning and took advantage of it," the American said.